1. Field of the Invention
The present invention relates to a method and system of velocity control for an automatic welding apparatus. More particularly, it relates to a method and system of velocity control for an automatic welding apparatus, wherein position control is effected while making linear interpolations between two points which are indicated by, for example, the rectangular coordinate system.
2. Description of the Prior Art
The position control devices heretofore used in automatic welding apparatuses may be divided broadly into two types, the PTP (Point To Point) type and the CP (Continuous Path) type. The PTP type position control device is simple in construction and inexpensive but its operation is not continuous. On the other hand, the CP type position control device is continuous in its operation but is complicated in construction and difficult to operate. Therefore, the PTP type position control device is being improved for practical use. In such a PTP type position control device, linear interpolations are necessary to effect a linear movement which is not along control axes (X, Y, Z) in, for example, the rectangular coordinate system.
For example, in conventional NC (Numerical Control), various linear interpolation systems are known, including the MIT system, the DDA system and the algebraic operation system. Any of these systems effect feed control along a straight line connecting two indicated points while correcting errors or deviations from said straight line, and the position control effected is accurate. However, the control operaton or program therefor is complicated and it tends to be more difficult if feed speed has to be controlled accurately.
As for a method of controlling a controlled object to a specified feed speed, there is a method using a tape which specifies the reciprocal of the time required to travel a specified distance (or this reciprocal multiplied by a suitable coefficient), the (feed speed coefficient (FRN). According to such method which specifies F or FRN or a tape, although the feed speed is accurate, the computation involved is troublesome.
Therefore, there is another method in which a numerical value specifies a feed speed, such feed speed itself being directly specified, and a suitable circuit in the control device is used to effect pulse distribution to assure that the feed speed will be exactly as specified. However, such method which specifies an absolute velocity on a tape encounters the problem of lack of versatility in that different tapes serving as programs have to be used for different operations.
As described above, the interpolation process can be carried out by a computer, for example, but the recent progress in the technology of semiconductors has made a single chip type microprocessor or microcomputer available. However, such microprocessor or microcomputer is limited in its computing function and generally its computing speed is low. Therefore, the use of a microprocessor or microcomputer for the purpose of reducing the cost of the apparatus would lead to the problem of the processing rate being low due to its low computing speed.
In the conventional PTP control in an automatic welding apparatus, the controlled object (for example, the torch) is stopped at each indicated point. Therefore, if it is desired to effect a nearly curvilinear control operation by increasing the number of indicated points, then the number of stop points increases, thus resulting in a poor welding finish. To overcome this difficulty, it would be thought advisable to cut off the welding current whenever the control object is stopped. In that case, however, the welding finish would not always be satisfactory while the construction would be further complicated and the welding operation would involve a substantial loss of time.
Heretofore, for example, in the case of arc welding, an automatic welding machine has been realized. The welding capacity of the torch in such arc welding is limited to a certain value, and hence, when the plate thickness of an object to be welded (workpiece) is increased, it is impossible to achieve the complete penetration of the melt into the interface in the case of butt welding.
Therefore, as in known in the art, with consideration given to the conditions including the plate thickness and welding torch capacity, wearing has been resorted to for better welding results. In such a conventional automatic welding machine which performs weaving (a welding machine which causes the welding torch to automatically trace a welding line in a plane), the welding torch is mechanically swung or moved to and fro across the welding line, or a rotating magnetic field is applied to the arc to swing the welding point. However, as described above, the prior art encounters various problems:, the presence of mechanical inertia; the additional space required to install attachments (including mechanical swing means and magnetic field applying means) around the welding torch; the increased weight of the apparatus; and the difficulty which arises in connection with changing the weaving conditions. Further, in an automatic welding machine using a polar coordinate system type robot, weaving control can be effected by interpolations made by a control circuit, but there arises a new problem is that this control circuit is complicated.
It has been proposed to use the rectangular coordinate system to effect weaving control. In such weaving control system of interest, there remains the problem that the direction of a welding line (that is, the direction of movement of the torch) should be parallel with one of the axes X, Y, Z of the rectangular coordinate system. This means that in fixing a workpiece, very accurate positioning is required, and moreover, a complicated workpiece needs to have its position changed for each welding operation, resulting in the drawback that continuous weaving cannot be carried out in different directions.
In preparing a program for an automatic welding apparatus, as described above, the operator tends to be absorbed in position instructions, thus forgetting other instructions. As a result, mistakes in program teaching often occur. In order to preclude faults otherwise caused by a malfunction due to such mistakes, it is desirable to check on errors at the time of program teaching.
In positioning a controlled object in such a automatic welding apparatus, unforseeable accidents often occur in which the controlled objects collides with other objects, often damaging machine parts. Such collision accidents occur most frequently especially in connection with teaching at the time of playback because the manual operation thereof tends to be carried out in a wrong way.